Justification of Red List Category
This newly split species is listed as Endangered because it occupies a very small range when breeding, nesting on only a very few islands and islets, and is inferred to be experiencing on-going decline owing mainly to the impacts of invasive mammalian predators. If it is found to be breeding at more than five locations, the species may warrant downlisting to a lower threat category.
It is estimated to number fewer than 5,000 breeding birds (Birt et al. 2012), which are presumed to form one sub-population. Until further data are obtained, an estimate of 5,000 mature individuals is used in this assessment, which is assumed here to equate to c.7,500 individuals in total.
A number of colonies have been extirpated by introduced predators and alien invasive species continue to threaten some colonies. A number of other threatening processes are on-going, the combined effects of which are suspected to be causing a moderate decline in this species's global population.
Synthliboramphus hypoleucus breeds along the west coast of Mexico, on the three San Benito Islands, Guadalupe Island and at least two associated offshore rocks (Birt et al. 2012, Chesser et al. 2012). Cat predation is thought to have caused the extirpation, or at the very least, significantly reduced the population on the main island of Guadalupe (Keitt et al. 2006), which is considered the most important historical site for the species (B. Tershy in litt. 2007). Other likely former breeding colonies (Cedros, Natividad, Asunción and San Roque) are thought to have been extirpated by invasive animals (B. Keitt and D. Whitworth in litt. 2003). Breeding is unconfirmed on San Martín Island, Baja California, and San Clemente and Santa Barbara islands, California (Chesser et al. 2012). The species winters offshore, with presumably the majority of the population remaining in the vicinity of the breeding range along the western coast of Baja California (Chesser et al. 2012). Some post-breeding birds disperse north to the coasts of California and Oregon, U.S.A. (Gaston and Jones 1998).
It nests on steep sea-slopes, canyons and cliffs with a sparse cover of herbaceous and shrubby plants (Drost and Lewis 1995). It is a generalist predator, and may exploit higher prey concentrations around pelagic convergence lines (Hamilton et al. 2004). Nesting persists through mid-June with peak nesting from late March to late April (Jones et al. 2005). Clutches consist of two eggs laid approximately eight days apart with replacement of lost clutches unusual. Incubation takes c. 34 days (Jones et al. 2005).
Invasive mammalian predators represent the most important threat to this species. Historically, cats Felis catus are thought to have extirpated Murrelets from several islands and is currently a significant barrier to recolonization and recovery (Keitt 2005, B. Tershy in litt. 2007). Cats likely extirpated, or at the very least, significantly reduced the population on the main island of Guadalupe (B. Tershy in litt. 2007). Substantial numbers of cat-killed Murrelet cadavers have been identified on Baja California islands (Keitt 2005). Introduced House Rats Rattus rattus represent another major threat; in 2002, 96% of artificial nests were depredated on control islands (rats present) with rats accounting for most of the predation, Nest predation on the treatment island (rats eradicated) was significantly lower with only 8% of artificial nests depredated, mostly by endemic Deer Mice Peromyscus maniculatus (Jones et al. 2005). Predation levels by Deer Mice appear to be low enough to not have any significant impact on the species (USFW 2016). In 2003, following rat eradication on the remaining islands (Middle and West Anacapa Islets), nest predation was reduced from 96% in 2002 to 3% in 2003 (note: these studies were done on Xantus' Murrelet Synthliboramphus hypoleucus scrippsi which is commonly lumped with Guadalupe Murrelet [Nettleship and Kirwan 2018] and, although it is not confirmed whether the Guadalupe Murrelet breeds on the studied islands, impacts are likely comparable).
Alcids appear to be extremely susceptible to oiling and often comprise the bulk of seabird mortality from oil spills in western North America. The threat of oil pollution to (Xantus’) Murrelets in the Southern California Bight has risen substantially since the early 1960s with increased oil tanker traffic. From 1963 to 1989, 35 offshore oil platforms were built in federal and state waters (eight of which were removed between 1974 and 1996). Examples of past oil spills within the species’ range include the 1986 Apex Houston barge oil spill and the 1969 Santa Barbara oil spill. In addition to direct mortality from oiling, the species is at risk from chronic oil contamination, with high chronic oiling of seabirds occurring in the Santa Barbara Channel and in offshore waters off central and southern California (Carter et al. 2000).
At present, organochlorine pollutants are considered not to have a significant effect on (Scripps’) Murrelets breeding at Catalina, but may have caused reduced reproduction or population size in the past, especially between the 1940s and 1970s, prior to the end of production and dumping of DDT in southern California waters (Drost and Lewis 1995, Whitworth et al. 2014). Compared to pre-1947 conditions, recent studies of (Xantus’) Murrelet eggs collected in 1992 have shown low levels of egg-shell thinning (i.e. 1.5%) and pollutants (i.e. 0.86 ppm total DDT, 0.35 ppm total PCB), compared to other seabird species (Burger and Fry 1993, Kiff 1994). Current effects from organochlorine pollution on related species of Murrelets appear to be low (Carter et al. 2000).Climate change and bycatch mortality may pose additional risks to the population, but are apparently not driving any significant declines at present. Temperature extremes may impact the species through food resource loss in El Niño years, however impacts on Synthliboramphus spp. appear modest due to their generalist prey (USFWS 2016). Gill-net fisheries are well known for incidental bycatch of alcids (Nettleship et al. 1984, DeGange et al. 1993) and entanglement in gill-nets may kill small numbers of Murrelets in the Southern California Bight, but is unlikely to be causing significant mortality levels (Carter et al. 2000). Bycatch was considered to currently operate at a very low impact level on Guadalupe Murrelet (USFWS 2016), and most is likely to occur in small-scale gill-net fisheries.
Conservation Actions Underway
The most important remaining conservation action is to eradicate cats from Guadalupe Island. Guadalupe Island has been declared a Biosphere Reserve (thanks to Conservación de Islas) (B. Tershy in litt. 1999). The remaining Mexican islands with current or former breeding colonies are either in existing biosphere reserves (Natividad, Asunción and San Roque) or in a proposed new biosphere reserve. This is the first step to regulating tourism and the more damaging impact of commercial fishers (B. Tershy in litt. 1999). A pilot habitat restoration for the species has begun on Santa Barbara Island (Wolf 2008).
Text account compilers
Harding, M., Martin, R., Miller, E., Sharpe, C.J., Symes, A., Taylor, J., Butchart, S., Calvert, R., Capper, D., Bird, J., Anderson, O., Fjagesund, T., Benstead, P., Gilroy, J.
Whitworth, D., Keitt, B., Tershy, B., Wolf, S.
BirdLife International (2021) Species factsheet: Synthliboramphus hypoleucus. Downloaded from http://www.birdlife.org on 16/10/2021. Recommended citation for factsheets for more than one species: BirdLife International (2021) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 16/10/2021.